Method of and system for controlling grinding mills

ABSTRACT

A grinding mill system including a grinding mill, a motor driven feeder for supplying material to the feed end of the mill, a classifier, means for carrying mill production in a gaseous stream, such as air to the inlet of the classifier for the separation of oversize material, a duct for returning oversize material to the feed inlet of the mill and means associated with the duct, including a rotary position transmitter responsive to variation in the rate of flow of oversize or reject material through said duct and means for controlling the feeder motor in response to actuation of the rotary position transmitter and variations in the rate of flow of reject material from the classifier to the feed inlet of the mill. The control system may include means such as a microphone responsive to variations in the sound of the mill.

United :States Patent Root [54] METHOD OF AND SYSTEM FOR CONTROLLING GRINDING MILLS [451 Sept. 12, 1972 Primary Examiner-Granville Y. Custer, Jr. Attorney-Pennie, Edmonds, Morton, Taylor and Adams [57] ABSTRACT A grinding mill system including a grinding mill, a motor driven feeder for supplying material to the feed end of the mill, a classifier, means for carrying mill production in a gaseous stream, such as air to the inlet of the classifier for the separation of oversize material, a duct for returning oversize material to the feed inlet of the mill and means associated with the duct, including a rotary position transmitter responsive to variation in the rate of flow of oversize or reject material through said duct and means for controlling the feeder motor in response to actuation of the rotary position transmitter and variations in the rate of flow of reject material from the classifier to the feed inlet of the mill. The control system may include means such as a microphone responsive to variations in the sound of the mill.

6 Claims, 4 Drawing Figures PATENTEDSEP 12 I972 SHEET 2 OF 2 #AJQ ATTORNEYS m T N E V m William E. Roof w W I I METHOD OF AND SYSTEM FOR CONTROLLING GRINDING MILLS BACKGROUND OF THE INVENTION 1. Field of the Invention I The present invention relates to improvements in the control of the operation of grinding mills based on variations in conditions of the operation of the mills.

2. Description of the Prior Art Mill control systems are well known in the art, and some of them may be illustrated by reference to U. S. Pat. No. 2,381,351, which proposes controls based on various features of operation of a mill. For example, it is quite conventional to make use of a microphone located adjacent the mill itself which produces an acoustical signal so that the sound of the mill is made use of as an indication of the grinding conditions carried out therein. The signal is sometimes coupled with other signals, but it is utilized for cutting off the electric power to the mill itself or to the motor or motors driving the feeder or feeders.

The patent discloses other types of controls for' stopping the feeder motor or motors, for example, variations in the differential air, pressure of a fan used for the circulation of mill production and the use of a control to stop the feed motor in response to flow of oversize material from a classifier in excess of normal amounts returned to the mill.

' More recent US. Pat. Nos., such as 2,766,939 and 2,766,941 disclose control systems which involve the use of a microphone responsive to the sound produced by the mill. Some of these systems are rather complicated and that disclosed by the '939 patent includes the use of a photoelectric apparatus arranged to view the density of the finely-divided product of the mill as it is carried away through a vertical conduit to a classifi- The present invention has for its primary object the simplification of a mill control system which will be efficient and effective without many of the complications heretofore used. The invention overcomes these objections and provides a control system which is directly and automatically responsive to a continuing function of the mill system.

SUMMARY OF THE INVENTION The present invention relates to improvements in grinding mill systems for grinding various solid materials, such as coal, ores and other minerals, the system including; a grinding mill, a motor driven feeder for the mill, a classifier for classifying the product of the mill, duct for conveying the classifier reject or oversize ground material from the classifier to the feed inlet of the mill, and means continually responsive to variations in the rate of flow of oversize material through the duct for in turn varying the rate of operation of the feeder.

The control imposed on the feeder is such that as the rate of flow of oversize material increases or decreases, the rate of feed by the feeder to the mill is respectively decreased or increased. The means responsive to the variations in the rate of flow of oversize material in the duct means advantageously comprises a member located in the duct means and arranged to gradually change its position in the duct in response to variations in the stream of oversize material flowing through the duct means. In a preferred construction the means associated with the duct and responsive to changes in the stream of oversize material comprises a rotary position transmitter arranged to produce an electric signal which is made use of in connection with necessary control elements to vary the feed rate of the feeder.

In an advantageous embodiment of the invention, the control system includes a microphone or sound responsive means located adjacent the mill and responsive to the true changes in the mill operation as evidenced by variations in the sound of the mill. This general type of control is known but in this invention the microphone is utilized through suitable conditioning and amplifying units to produce a clean signal representative of-actual changes in the sound produced in the mill, which changes correspond to the operating conditions of the mill. The signal pr'oducedby the microphone is used to reduce the feed output to practically zero if the sound level picked up by the microphone circuit triggers a monitor switch connected into the feeder control. The microphone circuit also advantageously includes an alarm relay to signal the operator that themill system has developed some problems which require attention. When the response to the microphone reaches the limit referred to, the system advantageously includes means which may take the feeder off automatic control.

In a preferred embodiment of the invention, the signal produced by the rotary position transmitter responsive to variations in the rate of flow of oversize material is sent through a series of control elements including a signal conditioning unit, a transducer delivering a controller input signal, and a recorder controller which finally is connected into the feeder motor control components. When the microphone is included as a part of the control circuit, which is preferable, the

microphone output signal is sent through a series of units including a signal conditioning unit, a transducer, a ratio controller and a monitor switch unit, from which the microphone signal for slowing the feeder to substantially zero is sent to the previously mentioned recorder controller or combined with its signal and then sent to the feeder motor control components.

The features of the invention described above are described more in detail hereinafter in connection with the embodiments illustrated in the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the diagrammatic showing in FIG. 1, the conventional features of the grinding apparatus include a ball mill l0 rotated by a motor 12 and supplied with the material to be ground by a feeder 14 which delivers the material at a predetermined controlled rate into an inlet chute 16. The ground material produced in the mill is delivered into a fiuidizing feeder 18, with the very coarse material having been retained in the mill by a screen at the discharge end of the mill. Drop-outs, such as tramp iron, leave the feeder 18 by an outlet 20.

The coarse and fine ground material is carried in an air or gas stream supplied from a blower 22 so that the coarse and fine materials are picked up and delivered through a duct 24 into a classifier 26.

I The ground material is classified in the classifier 26 to provide an air stream containing the fine material, delivered through an outlet 28 and sent to a cyclone separator. The cyclone separator may supply recirculating gas for the blower 22. The classifier reject or oversize ground material is delivered through a downwardly inclined duct 30 to the mill inlet chute 16, so that it joins the normal amount of material supplied to the mill by the feeder 14.

Since the primary control of the mill during its normal operation is provided by changes in the rate of oversize ground material flowing through the duct 30, a section 32 of-the duct is provided with a pivoted paddle or vane structure 34 which is actuated in response to variations in the flow rate in the duct 30. FIGS. 2 and 3 show the details of the structure and mechanism located in the duct section 32. The paddle or vane 34 actuates a shaft 36, which in turn controls the movement of the rotary position transmitter 38 shown in FIGS. land 4. The output electrical signal of the rotary position transmitter is delivered through a line 39 to a signal-conditioning unit 40, which is adapted to filter out stray signals and pulses so that the conditioned signal is transmitted through a transducer 42 which, for example, has anoutput signal of from 4 to MA. The signal delivered by the element 42 is taken to a recorder controller 44, the control signal from which is transmitted through a line 45 to the feeder drive mechanism including an electric powersupply 46 which is connected to the feeder drive motor 48. The voltage supply to the element 46 is indicated by an arrow 50 may comprise in a particular instance 460 volts in a three line delivery at a frequency of 60.

The feedrate is proportional to the speed of the motor 48 and tachometers 52 are illustrated as respectively connected into the unit 46 to provide feed back for speed regulation and into a recorder 54 for the feed rate to the mill.

While the operation of the mill is effectively controlled by the primary signal from the controller 44, there is a distinct advantage in providing means including a signal from a microphone 56, responsive to the variations in the sound of the mill 10, which may have the effect of reducing the output of the feeder to nearly zero. In FIG. 1, the microphone or sound pick up 56 is located adjacent the mill in the usual way, the signal from which goes to a signal conditioning unit 58 connected into a transducer 60 having the same rating as the transducer 42. The signal from the element 60 is sent directly to a monitor switch 62 and through a ratio controller 64 as indicated. The monitor switch 62 includes meter relay switches to a manual loader 66 on low limit and is adapted to deliver a signal to the feeder controller 44 as indicated so that it can in effect provide an override to the controller 44 and in case of emergency reduce the feeder output to an extremely low value.

In an alternate arrangement, shown in dot and dash lines in FIG. 1, the signal from the transducer 60 is sent through a line 61, to a monitor switch 62' which receives the signal from the controller 44 through a line A signal is sent directly from the monitor switch 62 to the feeder controller 46 through a line 63. p

The arrangement is such that if the sound level of the mill reaches a predetermined limit levelit triggers the monitor switch 62 or 62 to switch in a manual loader 66 or 66, which then is connected into the controller 44 output circuit. The signal from the manual loader 66 or 66 when connected in by the signal supplied to the monitor switch 62, will cause the feeder to reduce its output to almost zero.

While not shown in the drawing, a basset switch may be provided at the controller 44 which is used to activate an alarm relay to in turn signal the operator that the mill has gone off automatic control and that problems exist somewhere in the system.

The sound pick up and amplifier elements referred'to above are preferably provided with a line voltage'for the amplifier itself at 120 volts A. C. at 60 Hz. The input source is the transducer 60 having a level of from 0 to millivolts PP A. C. at normal 1 KC frequency. The output is to have a level offrom 0 to 50 M. V. D. C. at 100ohms Z nominal.

The details of the structure provided in the duct section 32 and its relation to the rotary position transmitter 38 is illustrated in FIGS. 2 & 3 and 4. The section 32 of the duct 30 as illustrated in cross-section in FIG. 3 and in FIG. 2, comprises interior walls 68, extending lengthwise in spaced relation in the lower part of the duct section limiting the flow to a central flow space 70. In FIG. 3 the vane or paddle 34 is illustrated as a T-shaped structure, the lower portion of which operates in the space 70 floating on the stream of oversizematerial. The paddle or vane 34 is fixed to the pivot shaft 36 extending across the duct section 32, one

. projecting end of which is connected by a coupling 37 to an extension of the shaft-36in the rotary position transmitter 38 as shown diagrammatically in FIG. 1. The vane 34 is inclined downwardly in the direction of flow of oversize material and is swingable through an arc depending upon variations in the rate of flow of the oversize material through the space and engaged by the lower portion of the inclined vane or paddle 34.

FIG. 4 shows the mechanism of the transmitter 38 attached to the shaft 36 outside the duct section 32. In this view, the shaft 36 is shown with a lever biased by a spring 76 in a direction to load the vane 34, so that it is pressed lightly against the stream of material flowing through the space 70. As the shaft 36 rotates back and forth, it actuates a pulley 78, connected by a stainless-steel-braided cable 80 to a pulley 82 giving a 2 to l step-up ratio. The pulley 82 is rotated back and forth imparting a rotating motion to a potentiometer 84. The potentiometer is arranged in a known manner to provide an output signal, which is varied by the rotating motion to provide a variable output signal sent to the signal conditioning unit 40 byline 39 which comprises the two leads leaving the transmitter 38, as shown in FIG. 4.

The potentiometer 84 in the rotary position transmitter 38 may be of known type having a value of 1K, including a resistance taper adjustment. The current leading to the unit 40 may be from 4 to MA. D.C., one line of which may go through a damping circuit.

1 claim 1. In a grinding mill system of the type utilized for grinding various solid materials and including a grinding mill, a motor driven feeder for supplying material to the mill, a classifier for classifying the ground product of the mill and a duct for conveying the classifier reject or oversize ground material from the classifier to the feed inlet of the mill, wherein the improvement comprises:

a. means mounted on the duct for oversize ground material responsive to variations in the rate of flow of oversize solid material through the duct to the feed inlet of the mill,

b. said responsive means includes a pivot shaft, a vane fixed to the pivot shaft and having a movable portion overlying and lightly resting on the stream of oversize solid material flowing through the duct,

c. means responsive to the movement of said vane and shaft caused by variations in the rate of flow of the oversize solid material in the duct and actuated by the shaft for producing a variable electrical signal for controlling the rate at which the feeder delivers solid material to be ground to the inlet end of the mill to decrease the feed rate as the rate of flow of oversize solid material increases and to increase the feed rate as the rate of flow of oversize solid material through the duct decreases, and wherein the means for producing said signal comprises a rotary position transmitter, a potentiometer in the transmitter for producing an output signal, and means connected to the shaft for actuating the potentiometer.

2. A system as claimed in claim 1, wherein the potentiometer is a rotary motion potentiometer provided with means for varying its signal output, and means connecting said shaft with said varying means.

3. A grinding mill system as claimed in claim 2, including means for resiliently biasing the shaft in a direction to apply a light pressure tending to move the vane against the stream of oversize material flowing through the duct.

4. In a grinding mill system of the type utilized for grinding various solid materials and including a grindin g mill, a motor driven feeder for supplying material to the mill, means for varying the feed rate of the feeder, a classifier for classifying the ground product of the mill and a duct for conveying a stream of the classifier reject or oversize ground material from the classifier to the feed inlet of the mill, wherein the improvement comprises:

a. means mounted in the duct for oversize ground solid material continuously responsive to variations in the rate of flow of oversize material through the duct to the feed inlet of the mill,

b. said responsive means including a T-shaped vane located in the duct with the base portion of the T overlying and lightly resting on the stream of material flowing through the duct and movable in response to variations in the rate of flow of oversize solid m terial in the duct, said duct at th positron of sai vane having a cross-sectiona area size material increases and toincrease the feed rate as the rate of flow of oversize material through the duct decreases.

5. In a grinding mill system of the type utilized for grinding various solid materials and including a grinding mill, a motor driven feeder for supplying material to the mill, means for varying the feed rate of the feeder, a classifier for classifying the ground product of the mill and a duct for conveying a stream of the classifier reject or oversize ground material from the classifier to the feed inlet of the mill, wherein the improvement comprises:

a. means mounted in the duct for oversize ground solid material continuously responsive to variations in the rate of flow of oversize material through the duct to the feed inlet of the mill,

. said responsive means including means overlying and lightly bearing against the stream of oversize material in the duct, and

c. means responsive to the movement of said overlying means caused by variations in the rate of flow of oversize solid material in the duct including a rotary position transmitter for producing an electrical signal, means for supplying said signal in succession through a signal conditioning unit, a trans- .ducer, and a controller and to the means for controlling the feed rate of the feeder delivering material to be ground to the inlet end of the mill to decrease the feed rate as the rate of flow of oversize material increases and to increase the feed rate as the rate of flow of oversize material through the duct decreases.

6. A grinding mill system as claimed in claim 5, including a sound responsive means located adjacent the mill for indicating the variations in the operative conditions of the mill and means for sending a signal from the sound producing means through a transducer and monitor switch, and means for delivering the signal from the controller to the monitor switch and for delivering the resulting signal to the control means of the feeder.

decrease the feed rate as the rate of flow of over- 

1. In a grinding mill system of the type utilized for grinding various solid materials and including a grinding mill, a motor driven feeder for supplying material to the mill, a classifier for classifying the ground product of the mill and a duct for conveying the classifier reject or oversize ground material from the classifier to the feed inlet of the mill, wherein the improvement comprises: a. means mounted on the duct for oversize ground material responsive to variations in the rate of flow of oversize solid material through the duct to the feed inlet of the mill, b. said responsive means includes a pivot shaft, a vane fixed to the pivot shaft and having a movable portion overlying and lightly resting on the stream of oversize solid material flowing through the duct, c. means responsive to the movement of said vane and shaft caused by variations in the rate of flow of the oversize solid material In the duct and actuated by the shaft for producing a variable electrical signal for controlling the rate at which the feeder delivers solid material to be ground to the inlet end of the mill to decrease the feed rate as the rate of flow of oversize solid material increases and to increase the feed rate as the rate of flow of oversize solid material through the duct decreases, and wherein the means for producing said signal comprises a rotary position transmitter, a potentiometer in the transmitter for producing an output signal, and means connected to the shaft for actuating the potentiometer.
 2. A system as claimed in claim 1, wherein the potentiometer is a rotary motion potentiometer provided with means for varying its signal output, and means connecting said shaft with said varying means.
 3. A grinding mill system as claimed in claim 2, including means for resiliently biasing the shaft in a direction to apply a light pressure tending to move the vane against the stream of oversize material flowing through the duct.
 4. In a grinding mill system of the type utilized for grinding various solid materials and including a grinding mill, a motor driven feeder for supplying material to the mill, means for varying the feed rate of the feeder, a classifier for classifying the ground product of the mill and a duct for conveying a stream of the classifier reject or oversize ground material from the classifier to the feed inlet of the mill, wherein the improvement comprises: a. means mounted in the duct for oversize ground solid material continuously responsive to variations in the rate of flow of oversize material through the duct to the feed inlet of the mill, b. said responsive means including a T-shaped vane located in the duct with the base portion of the T overlying and lightly resting on the stream of material flowing through the duct and movable in response to variations in the rate of flow of oversize solid material in the duct, said duct at the position of said vane having a cross-sectional area fitting the base portion of the T-shaped vane, and c. means responsive to the movement of said overlying base portion of said T-shaped vane caused by variations in the rate of flow of oversize solid material in the duct for actuating the means for varying the rate at which the feeder delivers material to be ground to the inlet end of the mill to decrease the feed rate as the rate of flow of oversize material increases and to increase the feed rate as the rate of flow of oversize material through the duct decreases.
 5. In a grinding mill system of the type utilized for grinding various solid materials and including a grinding mill, a motor driven feeder for supplying material to the mill, means for varying the feed rate of the feeder, a classifier for classifying the ground product of the mill and a duct for conveying a stream of the classifier reject or oversize ground material from the classifier to the feed inlet of the mill, wherein the improvement comprises: a. means mounted in the duct for oversize ground solid material continuously responsive to variations in the rate of flow of oversize material through the duct to the feed inlet of the mill, b. said responsive means including means overlying and lightly bearing against the stream of oversize material in the duct, and c. means responsive to the movement of said overlying means caused by variations in the rate of flow of oversize solid material in the duct including a rotary position transmitter for producing an electrical signal, means for supplying said signal in succession through a signal conditioning unit, a transducer, and a controller and to the means for controlling the feed rate of the feeder delivering material to be ground to the inlet end of the mill to decrease the feed rate as the rate of flow of oversize material increases and to increase the feed rate as the rate of flow of oversize material through the duct decreases.
 6. A grinding mill system as claimed in claim 5, including a sound responsive means located adjacent the mill for indicating the variations in the operative conditions of the mill and means for sending a signal from the sound producing means through a transducer and monitor switch, and means for delivering the signal from the controller to the monitor switch and for delivering the resulting signal to the control means of the feeder. 